1. Field of Invention
The present invention relates to a dynamic pressure sensing structure and more particularly to a dynamic pressure sensing structure applied in a condenser microphone.
2. Related Art
In the trend of ‘smaller and lighter’ in the modem-day markets, micro-electro-mechanical system (MEMS) technologies have been developed to meet these requirements. These MEMS have the advantages of miniature-permitted, batch manufacturing, low cost of used materials and high added values and thus are deemed as the most promising products in the future.
Microphones are dynamic pressure sensors, which may sense very small variations like people's ears to sound. However, human's ears may react to only those sounds having specific frequency ranges owing to the physiological structures of people. Quite the contrary, the microphones with different structures may sense desired sounds with different frequency ranges. However, the traditional microphones generally presented in the market have their limitations in sensing sounds with relatively high frequencies, such as the sound of vibrations of machines of middle frequencies, the closing sound of the heart mitral valves, the sound of turbulence flow of blood in blood vessels and the sound created by the rubbing between bone and ligament.
Silicon crystal microphones are manufactured based on the MEMS technology, which may considerably reduce their manufacturing costs, further miniaturize their volumes and promote their sensitivities. Thus these microphones are quite superior to the traditional microphones. Accordingly, they may be qualified to be applied in industries, medical treatments and environmental protections and the like. The silicon crystal microphones may be roughly classified into piezoelectric microphones, piezoelectric/piezoresistive microphones and condenser microphones. Of the three types of silicon crystal microphones, the condenser silicon crystal microphones have become a main trend since they exhibit higher sensitivities and lower power dissipations.
The condenser microphone comprises a capacitor formed with two electrode plates disposed in parallel and having fillings of air or other insulating materials. The capacitor is connected to a positive end and a negative end of a battery at its two plates respectively to induce a capacitance C=ε0εr A/d, wherein ε0 is the dielectric constant in the vacuum, εr is the relative dielectric constant of the material disposed between the two plates with respect to the vacuum, A is the area of each of the plates and d is the distance between the two plates. In the capacitor, the distance between the two plates determines the charges stored. That is, the smaller the distance between the two plates is, the more the charges stored in the capacitor are. In a real operation, the principle of the relationship between the distance and the amount of the charges in the capacitor is relied on to obtain a sense output. Specifically, when a sound is directed towards the condenser microphone, an acoustic wave corresponding to the sound has an action on the air and thus the air is compressed, which results in vibration of a diaphragm in the microphone correspondingly. In response thereto, a variation of distance is occurred between the two plates of the capacitor and the charges in the capacitance correspondingly. A sense conversion circuit may acquire this varied capacitance and then a voltage is outputted.